1. Field of the Invention
The present invention is directed to a process of forming consolidated fully dense, near-net-shaped metal parts or objects by powder a metallurgy. More specifically, the present invention is directed to a powder metallurgy process which is capable of consolidating powders to form complex-shaped parts or objects within a straight walled metallic die with pressure supplied by a simple press.
2. Brief Description of the Prior Art
Near-net-shaped metal parts or objects may be produced by several processes, such as casting, forging, extrusion, cold or warm forming, and powder metallurgy. Certain types of objects or parts can be made only by one of the above-noted processes. The capabilities of the several near-net-shape processes overlap to a certain extent and therefore most parts or objects can be manufactured by more than one process. For this reason, in many cases a particular near-net-shape manufacturing process is selected for a certain part as a result of economic considerations, or because the selected process provides the more desirable mechanical and/or metallurgical properties in the finished part.
Powder metallurgy processes have relatively low requirements for energy and a high degree of material utilization, relatively short processing cycle time, and relatively low machining requirements. Primarily for these reasons, powder metallurgy (P/M) has been making advances into the markets served by other, more traditional, near-net-shape technologies, such as casting and forging. Moreover, relatively recently developed powder metallurgy techniques are capable of producing fully dense, chemically homogeneous, fine microstructured metal or ceramic parts, which may have superior overall qualities, at a lower manufacturing cost, than comparable more conventionally manufactured parts.
The most frequently used powder metallurgy process for making full density near-net-shaped parts is Hot Isostatic Pressing (HIPping). As is well known in the art, in the HIPping process, a powder part or compacted powder is subjected, at elevated temperatures, to equal pressure from every side, the pressure being transmitted by a pressurizing inert gas, usually argon. Typical conditions of the HIPping process range from 3 to 45 ksi (20 to 300 MPa) pressure (approximately 15 ksi, 100 MPa being average), and 895.degree. F. (480.degree. C.) to 3090.degree. F. (1700.degree. C.) temperature. The temperature, of course, depends greatly on the nature of the metal alloy which is being consolidated in the process. A review of the state-of-the-art of HIP processing, as applied to metal powders, is given by Peter E. Price and Steven P. Kohler in Metals Handbook, 9th Ed., Vol. 7, Hot Isostatic Pressing of Metal Powders, ASM, Metals Park, Ohio, pp. 419-443. U.S. Pat. Nos. 4,339,271, 4,359,336, and 4,379,725 describe, for example, processes relating to or comprising improvements of the above-summarized basic HIPping process.
The high cost of the pressure vessels and other equipment which is required for HIPping, canning of the compact before pressurization (to prevent oxidation and gaseous penetration of the consolidated product), the relatively long cycle time, and other factors make HIPping, overall, a costly process. Because of these and other disadvantages associated with HIPping, several alternatives to the HIPping process emerged during the recent years. Three of these alternatives, the so-called CERACON process, Rapid Omnidirectional Compaction (ROC), and the STAMP process are described by Lynn Ferguson in an article titled "Emerging Alternatives to Hot Isostatic Pressing", International Journal of Powder Metallurgy and Powder Technology, Vol. 21(3), 1985.
Each of the above-noted alternatives to HIPping "attempts to approximate the isostatic pressure conditions of HIP during consolidation of powder metal parts, while using conventional pressing equipment". Thus, in these alternatives the pressurizing gas in the HIP vessel is substituted for by a secondary pressing medium, which is typically ceramic material or carbon. In these alternative processes, the advancing top punch of a conventional press pressurizes the secondary pressure medium which transfers pressure to the workpiece. The result is consolidation of the workpiece under nearly isostatic conditions.
The STAMP process (one of the above-noted alternatives to HIP), for example, is used to produce billets and semifinished workpieces which are subsequently hot worked to shape. More particularly, in the STAMP process a conventional press is used to consolidate a powder mass contained in a can. The pressure of the press is transmitted through a secondary medium. The STAMP process is described in more detail, for example, in "Stamp Process", Metals Handbook, 9th Ed., Am. Soc. of Metals, Metals Park, Ohio, Vol. 7, pp. 547-550, 1984.
The STAMP process is not intended or capable of producing net-shaped or nearly net-shaped products. A probable reason for this lies in the fact that, under pressure, plastic deformation of the compacting powder body of the manufactured object occurs at rates and directions which is defined by the elastic/plastic deformation of the surrounding medium. The compressibility ratios of the powder of the object and of the medium are not equal. Therefore, after pressing under a given set of pressure and temperature conditions, the achieved final densities (expressed as percentage of theoretical density) of the two materials are not equal. In light of this, it will be readily understood that if, for example, during pressing in the STAMP process full density is achieved in the powder mass of the manufactured part but not in the pressurizing medium, then the fully densified part being incompressible (its density can no longer be increased) continues to deform in the direction of the weaker and perhaps more openly packed pressurizing medium. This, of course, leads to distortion of the manufactured part. Frictional differences between the powder of the manufactured part and the surrounding medium also have a distorting effect in the STAMP and like processes, probably for reasons which are similar to the reasoning elucidated regarding compressibility differences.
A net result of the foregoing and related effects is that in the STAMP and like processes of the prior art (which substitute a non-gaseous secondary medium for the pressurizing gas of HIP) variations in the several processing parameters affect the final shape of the consolidated part so that it is very difficult to hold close tolerances. (The processing parameters which affect the shape of the final part include pressure, temperature, temperature distribution, powder particle shape and distribution, and particle surface chemistry.)
Initial versions of the Ceracon process (another of the three prior art alternatives to HIPping), as disclosed in U.S. Pat. Nos. 3,356,496 and 3,689,259, were difficult to practice in terms of keeping dimensional control over the manufactured part. These processes were also too cumbersome. Improvements over the basic Ceracon process, such as, for example, the ones described in U.S. Pat. Nos. 4,499,048, 4,499,049, and 4,539,175, specify spherical ceramic particles, carbon, or their mixtures for the pressurizing medium in order to reduce interparticle friction. These patents do not address the need for uniform compressibility to achieve dimensional control so that when compressed powder compacts are pressed within the ceramic or carbon medium, final dimensions differ in all directions, most often unpredictably.
Another near net shape processing technique which is related to powder metallurgy is "powder forging". Powder forging involves the steps of (1) cold compaction of the metal powder; (2) sintering the compact; (3) heating the compact, and (4) forging the compact in a closed die typically maintained at approximately 600.degree. F. temperature. Although powder forging is a low cost, high "through-put" process, it is limited to manufacturing part shapes which can be made in a closed die. Powder forging also has the disadvantage of likely cracking in places where thick and thin sections are forged together.
None of the presently known powder metallurgy processes considered the importance of relative compressibility of the pressurizing medium and of the powder of the manufactured part. In other words, none of these processes, known to the present inventor, realized, in terms of the dimensions of the manufactured part, the importance of the phenomenon that, under pressure, compactions of the secondary pressure medium and of the powder of the part proceed at different rates. This, in the opinion of the present inventor, is one of the important reasons why the prior art powder metallurgy processes involving a secondary medium other than pressurized gas, fail to provide sufficient dimensional control of the manufactured part and therefore require excess machining stock on the surface to meet dimensional requirements.
The present invention provides a significant improvement over the prior art powder metallurgy processes in terms of dimensional control of the manufactured part and therefore in overall cost.